A diagnostic ultrasound system for capturing an ultrasound image of a object to be examined supplies a driving signal for transmission to an ultrasound transducer, emits an ultrasonic wave to the object to be examined, and reconstructs and displays an ultrasound image on the basis of the signal received at the ultrasound transducer.
As such a diagnostic ultrasound system, there is one that displays an elasticity image representing the hardness or softness of living tissue of a object to be examined. For example, an examiner manually compresses an ultrasound transducer whose ultrasound transmission and reception surface is in contact with the object to be examined against the object to be examined. The diagnostic ultrasound system obtains time-sequential images related to the living tissue when pressure is applied to the object to be examined. Elasticity data related to the living tissue is computed in correlation with the obtained time-sequential images, and an elasticity image is constructed on the basis of the computed elasticity data and is displayed.
More specifically, pressure sensors are provided on the back of a transducer element unit of the ultrasound transducer; the pressure applied to the ultrasound transducer by compressing the object to be examined is determined; and Young's modulus is determined to display an elasticity image. When a predetermined threshold of pressure is exceeded, a light emitting diode provided on the ultrasound transducer is emitted. Such a calculation method is described in Patent Document JP2003-225239A.
However, in the Patent Document, only Young's modulus is calculated by determining the pressure applied to the ultrasound transducer, and there is no mentioning of displaying the compression state on a screen. The operation of the ultrasound transducer to compress the object to be examined is carried out manually by the examiner. Therefore, when compressing the object to be examined, the compression direction may deviate from a predetermined direction due to tilting of the ultrasound transmission and reception surface. As a result, pressure may not be uniformly applied to the object to be examined. In other words, a non-uniform stress distribution may be generated in the living tissue of the object to be examined.
As an ultrasound transducer, for example, there is one whose ultrasound transmission and reception surface is curbed toward the living tissue side. When compression operation is carried out with such an ultrasound transmission and reception surface, the pressure in the compression direction may become the greatest because of the shape of the ultrasound transmission and reception surface. In other words, a non-uniform stress distribution may be generated in the living tissue of the object to be examined.
When a non-uniform stress distribution is generated in the living tissue, regions that are sufficiently compressed with predetermined pressure (hereinafter referred to as ‘adequate compression ranges’) and regions that are insufficiently compressed (hereinafter referred to as ‘inadequate compression ranges’) both exist in the living tissue in the visual field of the ultrasound transducer. When an elasticity image related to such living tissue is displayed, it is difficult to determined, for example, whether an image corresponding to an inadequate compression range is caused by non-uniformity of the stress distribution or caused by the hardness of the living tissue. Thus, an image corresponding to an inadequate compression range should not be referred to as diagnostic information. However, the process of identifying an adequate compression range and an inadequate compression range in a displayed image depends on the examiner's experience. For this reason, there is a demand for being able to objectively and quantitatively grasping a compression state of living tissue, such as the adequate compression range, the compression direction, and the compression angle.